1. Field of the Invention
The present invention relates to annealing and descaling of stainless steel sheet or strip to obtain synergistically the elimination of subsequent acid pickling, and, more particularly to ensure the formation of a thin uniform oxide scale layer to allow complete removal in low current density electrolyte.
2. Description of the Prior Art
In the manufacture of flat-rolled stainless-steel sheets and strip products, it is necessary to anneal or soften the material, subsequent to the cold-rolling operation. One very common annealing practice involves heating the steel in an oxidizing furnace atmosphere to a temperature that also causes the formation of oxide surface scale. Such heating, to a temperature typically in the range of 1450 degrees Fahrenheit to 2150 degrees Fahrenheit (788 degrees Celsius to 1177 degrees Celsius), depending upon the chemical composition of the steel being processed, is commonly done in a refractory-lined, gas-fired furnace containing an oxidizing atmosphere, and such treatment typically results in the formation on the stainless steel of an oxide scale having a thickness in the range of 4000 Angstroms to several microns. The scale must ultimately be removed before the product is considered ready for sale. To accomplish this, according to the prior art, any of several different descaling processes may be used, alone or in combination, and among these known processes there are (1) shot blasting following by acid pickling, (2) conditioning the scale by immersing the product in molten salt or subjecting it to an electrolytic treatment, followed by acid pickling, and (3) straight acid pickling. The acids commonly used in the pickling operation include sulfuric, nitric, and nitric-hydrofluoric combinations. The use of such acids is disadvantageous and costly, not only because the acids themselves are relatively expensive, but also because they are hazardous materials which necessitate the use of special handling techniques before, during, and after their use; moreover, because of environmental regulations, the disposal of waste pickle liquor presents costly problems.
The prior art also contains U.S. Pat. No. 4,363,709 which discloses the removal of oxide scale from the surface of a metallic body of stainless steel, particularly of the 300 and 400 series, with the use of a high-current-density electrolytic descaling process in a bath consisting of an aqueous solution containing about 15 to 25 weight percent of sodium sulfate, maintained at a temperature of at least 150 degrees Fahrenheit (65.6 degrees Celsius). According to the above-mentioned patent, the stainless steel is subjected as the anode to the action of a direct electric current for at least 10 seconds at a current density of at least 3 amperes per square inch (46.5 Amps/dm.sup.2). It was suggested that high chromium ferritic alloys may have to be thoroughly cleaned prior to annealing to avoid thick oxide scales, including chromium oxide.
U.S. Pat. No. 4,415,415 discloses a process for controlling oxide scale formation and descaling stainless steel. The oxide scale is developed in an oxidizing furnace atmosphere of controlled oxygen content and, thereafter, is conditioned electrolytically at a low density electric current of 0.1 up to 1.0 ampere per square inch.
The prior art contains, moreover, a group of patents which are concerned with the problem of providing heat to metal strip or sheet material by means of electrical induction. This prior art includes, for example, the following U.S. Pat. Nos. 4,054,770; 4,585,916; 3,444,346; 2,902,572; and 4,678,883. These patents may be considered as relating to such an electromagnetic induction process called "Transverse Flux Induction Heating", abbreviated "TFIH".
The prior art also contains U.S. Pat. No. 4,824,536 which discloses the processing of cold rolled stainless steel strip or sheet by providing annealing and subsequent descaling of the stainless steel by a combination of induction heating to an annealing temperature ranging up to 2300 degrees Fahrenheit (1260 degrees Celsius) causing the formation on the steel of scale having a thickness of up to 2000 Angstroms. The annealing process is followed by electrolytic descaling at high current density in an aqueous solution of 15 to 25 weight percent of sodium sulfate, maintained at a temperature of at least 150 degrees Fahrenheit (66 degrees Celsius) for a time sufficient to substantially entirely descale the steel. The oxide scale thickness was significantly less than 2000 Angstroms at the completion of the annealing operation, and specifically, was on the order of 700 to 1400 Angstroms and required a current density during the electrolytic descaling of at least 3 amperes per square inch. The induction annealing treatment followed by electrolytic treatment in this manner represented a notable advancement over the typical annealing operation in a gas fired atmosphere where oxidation of the stainless steel sheet or strip is extensive unless special measures are taken to provide an inert or reducing atmosphere during the annealing operation.
It has been found that induction heating produces an oxide scale which is relatively compositionally consistent and uniformly thick. Furthermore, it has been found that such a scale is removed quite effectively by electrolytic treatment. For example, the '536 patent disclosed that thinner scale thicknesses such as 700 angstroms occurred at lower annealing temperatures, such as 1850 degrees Fahrenheit (1010 degrees Celsius) whereas a thicker scale thickness of 1400 angstroms occurred at an annealing temperature of 2057 degrees Fahrenheit (1125 degrees Celsius). The widely varying scale thicknesses impose process variables to the electrolytic treatment that varies significantly as a function of the annealing temperature. Annealing temperatures on the order of 1950-2100 degrees Fahrenheit (1066-1149 degrees Celsius) are used for conventional gas-fired annealing processes.
It is believed that higher annealing temperatures will be needed for TFIH to achieve stainless steel metallurgical time-at-temperature requirements. Such temperatures may range from 1700 degrees Fahrenheit for ferritic and 1900 degrees Fahrenheit for austenitic up to 2300 degrees Fahrenheit, and more likely may range 2000 to 2300 degrees Fahrenheit.
The need therefore exists for a process that will allow scale removal by electrolytic treatment of an induction annealed stainless steel that produces only a thin uniform scale formation but also affects a savings through a materially lower current flux density than the heretofore required flux density of at least 3 amps per square inch. The process of the present invention further seeks to eliminate the need for follow-up acid pickling.
Accordingly, it is an object of the present invention to provide a process for treating stainless steel sheet or strip by initially cleaning the steel surface prior to annealing by transverse flux induction heating.